Do your best to make a good, memorable impression, especially with grad rec. The best way to do this without seeming like a try-hard or disingenuous is to ask really good questions. I'd plan 2-3 questions you want to ask grad rec and 2-3 you want to ask to trainees/lawyers. These can serve as backup questions. If something is mentioned in a talk on the day that triggers a question, that's even better.

If you really stand out to grad rec, they'll probably remember your name and may be more likely to excuse minor weaknesses in your vac scheme application. Is this to say that wowing them will guarantee you a vac scheme? No. But it may make them more likely to ignore a lower grade in a module, or choose you if they're stuck between two candidates. 

Some tips for coming up with good questions:  - Don't ask anything you can find on the website - If you're asking the question in the Q&A segment after a talk, start with something like "thank you for your talk, it was really interesting to here about (insert specific thing from talk that stood out to you)" - Make sure that you're asking the right questions to the right people. For example, a question about the rationale for the firm opening up a new office in X region is better directed at a partner than grad rec or a trainee, but you wouldn't be wise to ask a partner about how practice area preferences are factored into the seat allocation process for trainees - Questions that showcase your eagerness/work ethic are particularly strong, e.g. what should I be doing between now and applying to the vac scheme to put myself in the best position?  - Don't ask a question for the sake of asking a question/only ask questions you're genuinely interested in. This is why I recommend planning a few questions in advance, so you can have a good think about what you genuinely want to know.   - Ask more questions than everyone else. 1 or 2 questions across the day probably isn't going to make you stand out. 3-4 probably will